1. Field of the Invention
This invention relates to a motor vehicle comprising two driven axles, and an interaxle power train, which operatively connects the two driven axles and comprises an interaxle coupling for transmitting torque only when the two coupling members of the coupling rotate at different speeds, and a speed-changing transmission in series with the coupling, and also comprising a brake system for braking all wheels of the driven axles.
2. Description of the Prior Art
In a motor vehicle, particularly in a passenger car, the brake system is usually so designed that the adhesion is utilized to a higher degree at the front axle than at the rear axle so that the wheels of the rear axle will not be blocked before the wheels of the front axle as a blocking of the wheels of the rear axle would eliminate the directional control of the vehicle. That utilization of the adhesion at the wheels of both axles to different degrees involves different wheel slips of the two axles. In a motor vehicle which has two driven axles, which are operatively connected by a hydraulic friction coupling or any interaxle coupling which transmits torque only when the coupling members of the coupling rotate at different speeds, the different wheel slips of the two axles will result in a differential speed so that torque will be transmitted by the interaxle coupling. Because a braking will cause the wheels of the driven front axle to rotate with a larger slip, i.e., at a lower speed, than those of the driven rear axle, the torque will be transmitted by the interaxle coupling from the driven rear axle to the driven front axle. Owing to that transmission of torque, the adhesion at the wheels of the driven rear axle will be utilized to a higher degree and the adhesion at the front wheels will be utilized to a lower degree so that said degrees of utilization will approach each other. For a given adhesion, a careful control of the brake pressure will permit a higher retardation of a vehicle having two driven axles than of a vehicle having only one driven axle because in the former the wheels of the rear axle will be braked more strongly and the transmission of torque from the rear axle to the front axle will delay the blocking of the wheels of the front axle. In case of a blocking of the wheels of the driven front axle, the high differential speed will cause the hydraulic friction coupling or other interaxle coupling between the two driven axle to transmit a higher torque so that the adhesion at the wheels of the rear axle will immediately be overcome, at least one of the wheels of the rear axle will be blocked and the directional control will be lost. That riding and braking behavior of the vehicle will not be altered when the interaxle power train comprises also a speed-changing transmission, e.g., a planetary gear train, in series with interaxle coupling.
From U.S. Pat. No. 4,605,087 it is already known to provide a drive system which has a hydraulic friction coupling connected between the two driven axles and in which in case of a blocking of the front wheels a blocking of the wheels of the rear axle will be prevented by a clutch or an overrunning coupling, which is connected between the two axles, so that a reduction of the speed of the front wheels resulting from an application of the service brake will not be transmitted by the power train to the rear wheels. In that arrangement the advantages afforded by the transmission of torque between the two driven axles in case of a careful application of the brake are eliminated.